Secretary of State Mike Pompeo warned countries against easing pressure on North Korea on Friday, citing sanctions violations that have allowed Pyongyang to import crucial supplies.

The U.S. contends that Pyongyang is violating a sanctions cap on importing refined petroleum, through illegal ship-to-ship transfers. The U.S. shared photographic evidence and documents tracing the ships to China and Russia with the U.N. Security Council’s sanctions committee this month.